Preparation and electrochemical properties of lithium–sulfur polymer batteries

The lithium/sulfur (Li–S) batteries consist of a composite cathode, a polymer electrolyte, and a lithium anode. The composite cathode is made from elemental sulfur (or lithium sulfide), carbon black, PEO, LiClO₄, and acetonitrile. The polymer electrolyte is made of gel-type linear poly(ethylene oxide) (PEO) with tetra ethylene glycol dimethyl ether. Cells based on Li₂S or sulfur have open-circuit voltages of about 2.2 and 2.5 V, respectively. The former cell shows two reduction peaks and one oxidation peak. It is suggested that the first reduction peak is caused by the change from polysulfide to short lithium polysulfide, and the second reduction peak by the change from short lithium polysulfide to lithium sulfide (Li₂S, Li₂S₂). The cell based on sulfur has the same reduction mechanism as that of Li₂S, which is caused by the multi process (first and second reduction) of lithium polysulfide. On charge–discharge cycling, the first discharge has a higher capacity than subsequent discharges and the flat discharge voltage is about 2.0 V. As the current load is increased, the discharge capacity decreases. One reason for this fading capacity and low sulfur utilization is the aggregation of sulfur (or polysulfide) with cycling.     

Concentration measurements in lithium/polymer–electrolyte/lithium cells during cycling

We report on in situ and ex situ concentration measurements in lithium/polymer–electrolyte/lithium cells during cycling. We have used three different methods which give complementary results, in good agreement with theoretical predictions and previous concentration measurements by Raman confocal microspectroscopy. Our methods allow to obtain concentration maps in the electrolyte, in particular, when dendrites are observed: from these measurements, we can correlate the onset of dendritic growth with local concentration gradients.     

UV-cured methacrylic membranes as novel gel–polymer electrolyte for Li-ion batteries

In this paper, we report the synthesis and characterisation of novel methacrylic based polymer electrolyte membranes for lithium batteries. The method adopted for preparing the solid polymer electrolyte was the UV-curing process, which is well known for being easy, low cost, fast and reliable. It consists of a free radical photo polymerisation of poly-functional monomers: Bisphenol A ethoxylate (15 EO/phenol) dimethacrylate (BEMA) was chosen, as it can readily form flexible 3D networks and has long poly-ethoxy chains which can enhance the movement of Li⁺-ions inside the polymer matrix. The preliminary results reported here refer to systems where LiPF₆ solutions swelled the preformed polymer membranes.     

Preparation and performance of novel Li–Ti–Si–P–O–N thin-film electrolyte for thin-film lithium batteries

Novel Li–Ti–Si–P–O–N thin-film electrolyte was successfully fabricated by RF magnetron sputtering from a Li–Ti–Si–P–O target in N₂ atmosphere at various temperatures. XRD, SEM, EDX, XPS, and EIS were employed to characterize their structure, morphology, composition and electrochemical performances. The films were smooth, dense, uniform, without cracks or voids, and possessed an amorphous structure. Their room temperature lithium-ion conductivities were measured to be from 3.6 × 10⁻⁷ S cm⁻¹ to 9.2 × 10⁻⁶ S cm⁻¹, and the temperature dependence of the ionic conductivities fits the Arrhenius relation. This kind of electrolyte possessed good properties is a promising candidate material for solid-state thin-film lithium batteries.     

Novel electrospun poly(vinylidene fluoride-co-hexafluoropropylene)–in situ SiO2 composite membrane-based polymer electrolyte for lithium batteries

Composite membranes of poly(vinylidene fluoride-co-hexafluoropropylene) {P(VdF-HFP)} and different composition of silica have been prepared by electrospinning polymer solution containing in situ generated silica. These membranes are made up of fibers of 1–2 μm diameters. These fibers are stacked in layers to produce fully interconnected pores that results in high porosity. Polymer electrolytes were prepared by immobilizing 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) in the membranes. The composite membranes exhibit a high electrolyte uptake of 550–600%. The optimum electrochemical properties have been observed for the polymer electrolyte containing 6% in situ silica to show ionic conductivity of 8.06 mS cm⁻¹ at 20 °C, electrolyte retention ratio of 0.85, anodic stability up to 4.6 V versus Li/Li⁺, and a good compatibility with lithium metal resulting in low interfacial resistance. A first cycle specific capacity of 170 mAh g⁻¹ was obtained when the polymer electrolyte was evaluated in a Li/lithium iron phosphate (LiFePO₄) cell at 0.1 C-rate at 25 °C, corresponding to 100% utilization of the cathode material. The properties of composite membrane prepared with in situ silica were observed to be comparatively better than the one prepared by direct addition of silica.     

Synthesis and characterization of polyether urethane acrylate–LiCF3SO3-based polymer electrolytes by UV-curing in lithium batteries

The prepolymers of polyether urethane acrylate (PEUA) were synthesized from polyether polyol (polyethylene glycol (PEG) or polypropylene glycol (PPG)), diisocyanate (hexamethylene diisocyanate (HMDI) or toluene 2,4-diisocyanate (TDI)), and the caprolactone-modified hydroxyethyl acrylate (FA2D) using the catalyst (dibutyltin dilaurate (DBTDL)) by stepwise addition reaction. Lithium triflate (LiCF₃SO₃) was dissolved in PEUA prepolymers, and plasticizer (propylene carbonate (PC)) was added into prepolymer and salt mixtures. Then photoinitiator (Irgacure 184) was also dissolved in the mixtures. Thin films were prepared by casting on the glass plate, and then by curing the plasticized prepolymer and salt mixtures under UV radiation. Electrochemical and electrical properties of PEUA–LiCF₃SO₃-based polymer electrolytes were evaluated and discussed to be used in lithium batteries.     

Mechanochemical synthesis of α-Fe2O3 nanoparticles and their application to all-solid-state lithium batteries

α-Fe₂O₃ fine particles have been prepared by a mechanochemical process and a solution process. α-Fe₂O₃ nanoparticles with aggregates composed of the several tens nm primary particles were produced by the mechanochemical process. The nanoparticles were applied to the electrode as an active material for all-solid-state lithium batteries and the electrochemical properties of the cell were investigated. Typical charge–discharge curves, as seen in the liquid type cell using the α-Fe₂O₃ nanoparticles as an electrode were observed in the all-solid-state cell. The first discharge capacity of the cell of about 780 mAh g⁻¹ was, however, smaller than the capacity of a cell using α-Fe₂O₃ particles prepared by the solution process, which were monodispersed particles of 250 nm without aggregates. In order to develop electrochemical performance of all-solid-state batteries, it is important to use the electrode particles without aggregation which lead to the formation of good solid–solid interface between active material and solid electrolyte particles.C     

Bipolar polymer electrolyte interfaces for hydrogen–oxygen and direct borohydride fuel cells

Direct borohydride fuel cells (DBFCs) using liquid hydrogen peroxide as the oxidant are safe and attractive low temperature power sources for unmanned underwater vehicles (UUVs) as they have excellent energy and power density and do not feature compressed gases or a flammable fuel stream. One challenge to this system is the disparate pH environment between the anolyte fuel and catholyte oxidant streams. Herein, a bipolar interface membrane electrode assembly (BIMEA) is demonstrated for maintaining pH control of the anolyte and catholyte compartments of the fuel cell. The prepared DBFC with the BIMEA yielded a promising peak power density of 110 mW cm⁻². This study also investigated the same BIMEA for a hydrogen–oxygen fuel cell (H₂–O₂ FC). The type of gas diffusion layer used and the gas feed relative humidity were found to impact fuel cell performance. Finally, a BIMEA featuring a silver electrocatalyst at the cathode in a H₂–O₂ FC was successfully demonstrated.     

Sodium tungstate as electrolyte additive to improve high-temperature performance of nickel–metal hydride batteries

Sodium tungstate (Na₂WO₄) used as new electrolyte additive to enhance the high-temperature performance of Nickel–metal hydride (Ni–MH) battery is investigated in this paper. The effects of Na₂WO₄ on nickel hydroxide electrodes are investigated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and a charge/discharge test. It is found that the Ni–MH cell with the conventional KOH electrolyte containing 1 wt.% Na₂WO₄ additive exhibits higher discharge retention and better cycling performance than the cell without Na₂WO₄ additive at both 25 °C and 70 °C. These performance improvements are ascribed to the enhancement of oxygen evolution overvoltage and lower electrochemical impedance, as indicated by CV and EIS. The results suggest that the proposed approach be an effective way to improve the high temperature performance of Ni–MH batteries.     

Ultralife’s polymer electrolyte rechargeable lithium-ion batteries for use in the mobile electronics industry

Ultralife Polymer™ brand batteries for cellular phones as made by Nokia Mobile Phones Incorporated were introduced in July 2000. Characteristics of the UBC443483 cell and UB750N battery are described and related to the power and battery requirements of these cellular phones and chargers. Current, power, and pulse capability are presented as functions of temperature, depth of discharge, and storage at the cell level. Safety protection devices and chargers are discussed at the battery pack level, as well as performance in cellular phones under various wireless communication protocols. Performance is competitive with liquid lithium-ion systems while offering opportunity for non-traditional form factors.     

Synthesis and performance of lithium vanadium phosphate as cathode materials for lithium ion batteries by a sol–gel method

Monoclinic lithium vanadium phosphate, Li₃V₂(PO₄)₃, was synthesized by a sol–gel method under Ar/H₂ (8% H₂) atmosphere. The influence of sintering temperatures on the synthesis of Li₃V₂(PO₄)₃ has been investigated using X-ray diffraction (XRD), SEM and electrochemical methods. XRD patterns show that the Li₃V₂(PO₄)₃ crystallinity with monoclinic structure increases with the sintering temperature from 700 to 800 °C and then decreases from 800 to 900 °C. SEM results indicate that the particle size of as-prepared samples increases with the sintering temperature increase and there is minor carbon particles on the surface of the sample particles, which are very useful to enhance the conductivity of Li₃V₂(PO₄)₃. Charge–discharge tests show the 800 °C-sample exhibits the highest initial discharge capacity of 131.2 mAh g⁻¹ at 10 mA g⁻¹ in the voltage range of 3.0–4.2 V with good capacity retention. CV experiment exhibits that there are three anodic peaks at 3.61, 3.70 and 4.11 V on lithium extraction as well as three cathodic peaks at 3.53, 3.61 and 4.00 V on lithium reinsertion at 0.02 mV s⁻¹ between 3.0 and 4.3 V. It is suggested that the optimal sintering temperature is 800 °C in order to obtain pure monoclinic Li₃V₂(PO₄)₃ with good electrochemical performance by the sol–gel method, and the monoclinic Li₃V₂(PO₄)₃ can be used as candidate cathode materials for lithium ion batteries.     

Polymer Ni–MH battery based on PEO–PVA–KOH polymer electrolyte

An alkaline polymer electrolyte film has been prepared by a solvent-casting method. Poly(vinyl alcohol), PVA is added to improve the ionic conductivity of the electrolyte. The ionic conductivity increases from 10⁻⁷ to 10⁻² S cm⁻¹ at room temperature when the weight percent ratio of poly(ethylene oxide), PEO to PVA is increased from 10:0 to 5:5. The activation energy of the ionic conductivity for the PEO–PVA–KOH polymer electrolyte is 3–8 kJ mol⁻¹. The properties of the electrolyte film are characterized by a wide variety of techniques and it is found that the film exhibits good mechanical stability and high ionic conductivity at room temperature. The application of such electrolyte films to nickel–metal-hydride (Ni–MH) batteries is examined and the electrochemical characteristics of a polymer Ni–MH battery are obtained.     

Alkaline composite PEO–PVA–glass-fibre-mat polymer electrolyte for Zn–air battery

An alkaline composite PEO–PVA–glass-fibre-mat polymer electrolyte with high ionic conductivity (10⁻² S cm⁻¹) at room temperature has been prepared and applied to solid-state primary Zn–air batteries. The electrolyte shows excellent mechanical strength. The electrochemical characteristics of the batteries were experimentally investigated by means of ac impedance spectroscopy and galvanostatic discharge. The results indicate that the PEO–PVA–glass-fibre-mat composite polymer electrolyte is a promising candidate for application in alkaline primary Zn–air batteries.     

Endurance study of a solid polymer electrolyte direct ethanol fuel cell based on a Pt–Sn anode catalyst

The performance decay of a solid polymer electrolyte direct ethanol fuel cell (DEFC) based on a Pt₃Sn₁/C anode catalyst during an endurance test has been investigated. The effect of different cell shut-down procedures on the cycled behaviour of the DEFC has been studied. To get specific insights into the degradation mechanism, polarization and ac-impedance spectroscopy studies have been carried out. These analyses have been complemented by post-operation transmission electron microscopy and X-ray diffraction studies. The combination of these techniques has allowed to get information on recoverable and unrecoverable losses. This provides a basis for further improvement of DEFC components.     

Electrochemical properties of Li–Mg alloy electrodes for lithium batteries

Li–Mg alloy electrodes are prepared by two methods: (1) direct-alloying through the melting of mole percent specific mixtures of Li and Mg metal under vacuum and (2) the kinetically-controlled vapor formation and deposition (KCVD) of a Li–Mg alloy on a substrate. It is found that processing conditions greatly influence the microstructures and surface morphologies, and hence, the electrochemical properties of the Li–Mg alloy electrodes. When applying the KCVD technique, the composition of each prepared alloy is determined by independently varying the temperature of the molten lithium, the temperature of magnesium with which the lithium interacts, and the temperature of the substrate on which the intimately mixed Li–Mg mixture is deposited. Here, the required temperature for lithium induced Mg vaporization is more than 200°C below the magnesium melting point. The effect of these variable temperatures on the microstructure, morphology, and electrochemical properties of the vapor-deposited alloys has been studied. The diffusion coefficients for lithium in the Li–Mg alloy electrodes prepared by the KCVD method are in the range 1.2×10⁻⁷ to 5.2×10⁻⁷ cm² s⁻¹ at room temperature, two to three orders of magnitude larger than those in other lithium alloy systems (e.g. 6.0×10⁻¹⁰ cm² s⁻¹ in LiAl). These observations suggest that Li–Mg alloys prepared by the KCVD method might be used effectively to prevent dendrite formation, improving the cycleability of lithium electrodes and the rechargeability of lithium batteries as a result of the high diffusion coefficient of lithium atoms in the Li–Mg alloy. Li–Mg alloy electrodes also appear to show not only the potential for higher rate capabilities (power densities) but also for larger capacities (energy densities) which might considerably exceed those of lithiated carbon or Sn-based electrodes for lithium batteries.     

Rechargeable lithium battery employing a new ambient temperature hybrid polymer electrolyte based on PVK+PVdF–HFP (copolymer)

We describe here for the first time, our recent success in developing an ambient temperature Li⁺ conducting solid polymer electrolyte (SPE) using the concept of polymer alloying upon blending two thermoplastic polymers such as poly(vinylidene) fluoride-hexafluoropropylene (PVdF–HFP-copolymer) and poly(N-vinylcarbazole), PVK and achieved the room temperature electrolytic conductivity (σ_i) of 0.7×10⁻³ S/cm for a typical composition of PVdF–HFP copolymer/PVK blend mixed with EC/LiBF₄ molar composition. The ionic transference number of 0.49 was deduced from combined ac-impedance and dc polarization method. High-resolution optical microscopic examination revealed the disappearance of characteristic highly porous surface structure of PVdF–HFP matrix upon blending with PVK leading to the formation of resultant PVdF–HFP/PVK blend polymer alloy. The electrochemical stability of the polymer electrolyte membrane thus obtained was found to be stable up to ∼4.7 V versus Li/Li⁺. The new hybrid alloy polymer electrolyte membrane was found to exhibit good interfacial properties against lithium metal and thus, it was found to aid the room temperature operation as electrolytic membrane cum separator in all-solid state rechargeable lithium polymer test cell, LiCo_0.8Ni_0.2O₂/SPE/Li.     

Metal foam-supported Pd–Rh catalyst for steam methane reforming and its application to SOFC fuel processing

Pd–Rh/metal foam catalyst was studied for steam methane reforming and application to SOFC fuel processing. Performance of 0.068 wt% Pd–Rh/metal foam catalyst was compared with 13 wt% Ni/Al₂O₃ and 8 wt% Ru/Al₂O₃ catalysts in a tubular reactor. At 1023 K with GHSV 2000 h⁻¹ and S/C ratio 2.5, CH₄ conversion and H₂ yield were 96.7% and 3.16 mol per mole of CH₄ input for Pd–Rh/metal foam, better than the alumina-supported catalysts. In 200 h stability test, Pd–Rh/metal foam catalyst exhibited steady activity. Pd–Rh/metal foam catalyst performed efficiently in a heat exchanger platform reactor to be used as prototype SOFC fuel processor: at 983 K with GHSV 1200 h⁻¹ and S/C ratio 2.5, CH₄ conversion was nearly the same as that in the tubular reactor, except for more H₂ and CO₂ yields. Used Pd–Rh/metal foam catalyst was characterized by SEM, TEM, BET and CO chemisorption measurements, which provided evidence for thermal stability of the catalyst.     

Graphite–FeSi alloy composites as anode materials for rechargeable lithium batteries

Iron–silicon are prepared by annealing elemental mixtures at 1000 °C followed by mechanical milling. Graphite–Fe₂₀Si₈₀ alloy composites have been prepared by ball-milling a mixture of Fe₂₀Si₈₀ alloy and graphite powder. The microstructure and electrochemical performance of the composites are characterized by X-ray diffraction and an electrochemical method. The FeSi₂ matrix is stable for extended cycles and acts as a buffer for the active centre, Si. The Fe₂₀Si₈₀ alloy electrode delivers large initial capacity, but the capacity degrades rapidly with cycling. Fe₂₀Si₈₀ alloy–graphite composite electrodes, however, show good cycleability and a high reversible capacity of about 600 mAh g⁻¹. These composites appear to be promising candidates for negative electrodes in lithium rechargeable batteries.